Download i April 9th 2010 To: Dr. Julio Militzer Dr. Marek Kujath From: Team 12

April 9th 2010
To:
Dr. Julio Militzer
Dr. Marek Kujath
From: Team 12
Loading Apparatus for High Velocity Tissue Rupture
Geoff Beck
Ben Breen
Rachael Schwartz
Ruth Domaratzki
Re: Winter Term Report
April 9, 2010
Team 12 designed an apparatus to perform uniaxial tensile tests on biological materials.
The apparatus is capable of conducting tensile tests at rates of strain above those
currently attainable at the Dalhousie Department of Biomedical Engineering. The design
overcomes strain rate limitations inherent with the currently employed servohydraulically actuated model at the sacrifice of other features on the present device that
are not required in the current research being conducted by the client. Such features
include the ability to conduct biaxial tests. The enclosed document outlines our successes
and shortfalls in the construction of this device.
Team 12 – Loading Apparatus for High Velocity Tissue Rupture
Geoff Beck
Ben Breen
Rachael Schwartz
Ruth Domaratzki
i
MECH 4020 – Design Project II
Team 12
Loading Apparatus for High Velocity Tissue Rupture
Geoff Beck
Ben Breen
Rachael Schwartz
Ruth Domaratzki
Supervisor
Dr. M. Kujath
Client
Dr. M. Lee
i
Abstract
In fulfillment of a Mechanical Engineering degree, Team 12 (2009-2010) completed the
Design Project courses, MECH 4010 and MECH 4020, in the fall and winter semesters.
The team has created a loading apparatus for high velocity tissue rupture for the
Department of Biomedical Engineering. The device held a design criterion of fracturing a
specimen of bovine tendon (up to 2.5cm) at a strain rate of 1000s-1, recording force,
position, and velocity (vs. time). This report describes the design, construction, and
testing of this senior design project.
Our device when tested, was capable of reaching linear velocities of 7000 mm/s in the
specimen track, which would correspond to a strain-rate of 800s-1 for a typical sample.
The period of acceleration however covered a longer distance than was reasonable for the
client, so some additional modifications would be necessary before publication-quality
tests are capable from this device. All measurements were verified through the use of
high-speed camera imagery.
ii
Table of Contents
1
Introduction ............................................................................................................... 1
1.1
1.2
Background .................................................................................................................. 1
Tissue Mechanics.......................................................................................................... 2
2
Objectives................................................................................................................... 4
3
Generation of Alternatives ....................................................................................... 6
3.1
3.2
4
Selected Design .......................................................................................................... 8
4.1
4.2
5
Hopkinson Split Bar Apparatus ................................................................................. 6
Gravitational Impact Pendulum ................................................................................. 7
Design Evolution .......................................................................................................... 8
Design Selection Matrices .......................................................................................... 10
Problem Exploration .............................................................................................. 10
5.1
Modeling ..................................................................................................................... 10
5.1.1 LAHVTR Mock-Up................................................................................................. 10
5.1.2 Finite Element Analysis........................................................................................... 11
5.1.3 Rapid Prototyping .................................................................................................... 11
5.2
Testing ......................................................................................................................... 12
5.2.1 Biological Specimen Testing ................................................................................... 12
5.2.2 Solenoid Testing ...................................................................................................... 12
6
7
Calculations ............................................................................................................. 13
Final Design ............................................................................................................. 16
7.1
Drive Shaft Assembly................................................................................................. 16
7.1.1 Flywheel Design ...................................................................................................... 17
7.1.2 Main Shaft ............................................................................................................... 19
7.1.3 Gripping Mechanism ............................................................................................... 20
7.1.4 Damper .................................................................................................................... 20
7.1.5 Safety Shield ............................................................................................................ 21
7.2
Electrical Components............................................................................................... 21
7.2.1 Motor and Frequency Drive..................................................................................... 21
7.2.2 Measurement Systems ............................................................................................. 22
7.2.3 Solenoid Actuation .................................................................................................. 24
8
Implementation of Measurement Systems............................................................ 26
8.1
8.2
9
10
Safety ........................................................................................................................ 27
Testing ...................................................................................................................... 27
10.1
10.2
10.3
10.4
10.5
11
Calibration of LVDT ................................................................................................. 26
Calibration of Force Transducer .............................................................................. 26
Test Procedure............................................................................................................ 27
Validation Method ..................................................................................................... 28
Measurement Equipment Testing Results ............................................................... 28
High Speed Video Testing Results ............................................................................ 31
Time of Impact Analysis ............................................................................................ 34
Issues Encountered ................................................................................................. 35
11.1
Electrical and Mechanical Crosstalk........................................................................ 35
iii
12
Impact on Society .................................................................................................... 35
13
Life Cycle Analysis.................................................................................................. 36
14
Work Allotment ...................................................................................................... 36
15
Budget ...................................................................................................................... 36
16
Future Considerations ............................................................................................ 37
16.1
16.2
16.3
Control of Initial Sample Length. ............................................................................. 37
Prevent deformation of critical components............................................................ 38
Other Considerations................................................................................................. 39
17
Conclusions .............................................................................................................. 39
18
References ................................................................................................................ 40
19
Appendices ............................................................................................................... 40
Appendix A– Previously Signed Documents......................................................................... 40
Appendix B: Decision-Making Tables and Charts .............................................................. 44
Appendix C: Gantt Chart....................................................................................................... 46
Appendix D: Budget................................................................................................................ 47
Appendix E: User’s Manual ................................................................................................... 49
Appendix F: Engineering Drawings ...................................................................................... 55
List of Figures
Figure 1: Crosslinking of Collagen Fibers (Lee, 2010) ...................................................... 2
Figure 2: Strength and Stiffness Relationships for Materials (Lee, 2010) ......................... 2
Figure 3: Structural-Mechanical Relations in Soft Tissues (Lee, 2010) ............................. 3
Figure 4: Hopkinson Split Bar Apparatus ........................................................................... 7
Figure 5: Gravitational Impact Pendulum........................................................................... 8
Figure 6: Early depiction of LAHVTR ............................................................................... 9
Figure 7: Mock-up of Device ............................................................................................ 11
Figure 8: Rapid Prototype of Device ................................................................................ 11
Figure 9: Solenoid voltage vs Time (theoretical-red, experimental-blue) ........................ 13
Figure 10: Desired vs Anticipated Strain Rate ................................................................. 13
Figure 11: Final Design .................................................................................................... 16
Figure 12: Drive shaft Assembly ...................................................................................... 17
Figure 13: Flywheel .......................................................................................................... 18
Figure 14: Flywheel Design .............................................................................................. 18
Figure 15: Engagement Pin in Flywheel ........................................................................... 19
Figure 16: Exploded View of Drive Shaft Assembly ....................................................... 19
Figure 17: Exploded Grip Assembly ................................................................................ 20
Figure 18: Damper set-up ................................................................................................. 21
Figure 19 - Diagram of Safety Shield ............................................................................... 21
Figure 20: Frequency Controller....................................................................................... 22
Figure 21: Specimen Force Measurement ........................................................................ 23
Figure 22: Wheatstone Bridge Configuration ................................................................... 24
Figure 23: Initial Position of Contact Tooth ..................................................................... 25
Figure 24: Final Position of Contact Tooth ...................................................................... 25
iv
Figure 26: Calibration Graph for LVDT ........................................................................... 26
Figure 27: Calibration Graph for Force Transducer ......................................................... 27
Figure 28: Raw Signals Acquired During Testing ............................................................ 29
Figure 29: Processed Signals ............................................................................................ 29
Figure 30: Velocity vs. Time Analysis ............................................................................. 30
Figure 31: Documented Force vs. Displacement Characteristic for Sample .................... 30
Figure 32: Force-Displacement Characteristic Obtained .................................................. 31
Figure 33: Creating a reference system for high-speed video analysis. ........................... 32
Figure 34: High-speed video frames and feature tracking for 8mm sample at 500RPM
and 500 frames per second. ............................................................................................... 32
Figure 35: Mechanical Cross-Coupling ............................................................................ 35
Figure 36: Current control of sample length ..................................................................... 38
Figure 37: Cross-sectional view of spring loaded tooth. .................................................. 39
List of Tables
Table 1: Mechanical Properties of Collagen and Elastin (Lee, 2010) ................................ 3
Table 2: Content and Mechanical Properties of Soft Tissue Composites (Lee, 2010) ....... 4
Table 3: Design Requirements Check list ........................................................................... 4
Table 4: Biological Specimen Testing Results ................................................................. 12
Table 5: Displacement data for 500RPM trial with 8mm sample .................................... 32
Table 2: Displacement data for 1000RPM trial with 8mm sample .................................. 33
Table 5: Comparative Chart of Estimated to Actual Machining Hours............................ 36
Table 6: Comparative Graph of Estimated to Actual Budget ........................................... 36
v
1 Introduction
1.1 Background
The standard uniaxial tensile tension test reveals many fundamental properties of
materials. Properties of the material such as the ultimate tensile stress, toughness, failure
modes and failure location are often sought. The results of uniaxial tensile tests can be
applied through the application of Mohr's circle of stress and material failure theories
(such as Tresca's theory of stress) to alternative scenarios where the material is in
bending, torsion, under axial load or in combinations thereof.
It is known that the properties obtained from uniaxial tension tests are a function of the
strain rate used during a test. This phenomena is both true of metallurgical materials and
for biological substances. For biological substances especially, this change in property is
indicative of strain rate dependant molecular mechanisms that determine mechanical
behavior. As an example of these molecular mechanisms, it was noted by Willett et al.
(2007) that in a case involving high strain rate rupture of collagenous tissue, a large
amount of visible recoil of fibers were seen. These fibers were theorized to suggest a
build-up of elastic energy in a case where one would expect that sliding would dissipate
this energy.
There are many facets of equipment that enable standard uniaxial tensile tension tests.
The equipment must be highly rigid, it must be fitted with delicate instrumentation to
measure force, deformation, time, and provide visualization of the sample. In the present
case the equipment must enable a relatively high strain rate be achieving large velocities
in short periods of distance (and thus time).
The design presented in this report overcomes strain rate limitations inherent with the
currently employed hydraulically actuated model. The design overcomes this limitation at
the sacrifice of other features on the present device that are not required in the research
being conducted by the client. Such features include the ability to conduct biaxial tests.
Specifically, the Loading Apparatus for High Velocity Tissue Rupture (LAHVTR) was
designed to fracture a specimen of bovine tendon (up to 2.5cm) at a strain rate of 1000s-1,
recording force, position, and velocity (vs. time) as design criteria dictated.
This report outlines the progress of the team on the design, construction, and testing of
the device. The brainstorming sequence surrounding the flywheel design, the final CAD
drawings for the design, biological sample test results, a Gantt Chart, a final budget, and
a description of the progress of the design unto completion are outlined in the report.
Team 12 is comprised of 4 members: Geoff Beck, Ben Breen, Ruth Domaratzki, and
Rachael Schwartz. Our supervisor is Dr. Marek Kujath, and our client is Dr. Michael Lee
of the Dalhousie Biomedical Engineering Department.
1
1.2 Tissue Mechanics
Research into the mechanical behavior of tissue allows for better understanding of failure
mechanisms, which can influence treatment of such tissues. Research into impact loading
of tissue could provide valuable information on the behavior of the collagen and elastin at
a molecular level. Currently few institutes are performing such research.
Collagen is the most common protein in the human body, making up 60% of its dry
weight (Lee, 2010). Collagen fibrils are secured by means of both intra, and inter
molecular bonds (Figure 1).
Figure 1: Crosslinking of Collagen Fibers (Lee, 2010)
The crosslinking presented above increased fibril; elastic modulus, tensile strength, and
toughness (Lee, 2010).
Figure 2: Strength and Stiffness Relationships for Materials (Lee, 2010)
The properties of collagen are somewhat surprising in that its strength is on the order of
almost 103Mpa (which is similar to steel and aluminum alloy). Its stiffness however, is
2
lower (around 103Mpa) in comparison to other materials (steel is on the order of
105Mpa).
Figure 3: Structural-Mechanical Relations in Soft Tissues (Lee, 2010)
Elastin is a protein polymer similar to collagen, but with more elastic properties. The
mechanicals properties of both collagen and elastin are presented in Table 1.
Table 1: Mechanical Properties of Collagen and Elastin (Lee, 2010)
Researchers have been fascinated by the performance of composite tissues such as
tendon, ligament, skin, and arterial tissues. These tissues are composed of differing
percentages of both collagen and elastin, and therefore exhibit differing behaviors as
shown in Figure 3. The content and mechanical properties of some soft tissue composites
are presented in Table 2.
3
Table 2: Content and Mechanical Properties of Soft Tissue Composites (Lee, 2010)
2 Objectives
The design requirements for the apparatus were agreed upon and were presented within
the design memorandum as follows:
•
•
•
•
•
•
•
•
•
The device will be mounted on a table top with one face approximately 30x30 in.
Strain rates achieved will be on the order of 1000s-1
The device should function approximately 5 years.
The conditions of the test sample will be as close as possible to physiological
conditions (100% humidity at 37˚C).
The device will be designed so the operator has control of the extension rate.
The device will be designed for safe operation performed by trained individuals. A
shielding component will be incorporated if required.
Accompanying the device will be a comprehensive instruction manual.
All set deadlines and time requirements set out in the MECH 4010/4020 Design
Project Handbook will be met.
The device will attempt to provide data describing the force and displacement
against time for each trial.
The design memorandum, design agreement and memorandum are located in Appendix
A of this report.
Table 3: Design Requirements Check list
Design
Size
Strain rate
Loading
Life
Conditions of the test sample
Safety
Requirements
30 x 30 inches
On the order of 1000s-1
Minimum 1/100s
5 years
Physiological conditions
(100% humidity at 37˚C).
-Instruction manual
-Shielding component
Accomplished
Yes
No
Yes
No
Yes
4
Data
-
Force
Displacement
Yes
The team produced a memorandum document in which the agreed design requirements
were outlined. The client and Team 12, before the design process begun, signed the
document. The following subsections profile the design requirements and discuss the
level of our accomplishments.
Size
The device should be able to fit on a tabletop with one face approximately 30X30 cm. The
client presented this design requirement because he wanted to install the device on a
laboratory counter in the Biological Tissue Testing Lab. The final apparatus measures
23X22 cm.
Strain Rate
The strain rate achievable should be on the order of 1000s-1. The velocity of the moving
grip is modeled as instantaneous. A sample of the length 0.8cm was tested and the
velocity of the moving grip was measured to be 6.5 m/s. Using these variables, the strain
rate was found to be:
v
ε′=
lO
6. 5 m s
0.008m
ε ′ = 814 s −1
ε′=
The maximum strain rate reached by the LAHVTR is 81% that of the design requirement.
Loading
Achieve a minimum of 1ms loading. A confirmed loading was 4m seconds.
Lifetime
Should last approximately 5 yrs. The LAHVTR is created from sturdy components with
overall robust design considered in every developing decision. Spare critical components
were machined for the client. This design requirement was met.
Conditions of Test Sample
The conditions of the test sample will be as close as possible to physiological conditions
(100% humidity at 37˚C). Used spray bottle to keep sample at conditions while loading
and unloading sample
Control
The device will be designed so the operator has control of the strain rate. Using the
frequency controller and the stroboscope, the operator has control over the velocity of the
5
device and therefore, the operator has control over the strain rate. This design
requirement was met.
Safety
The device will be designed to be safely operated by trained individuals. A shielding
component will be incorporated if required. The device operated well within safe
operating conditions. The device is designed to be operated by an individual that is
trained by reviewing the operation and safety manual. Within this demographic, this
design criteria is met. It became apparent during the design process that it was necessary
to fit the LAHVTR with a polycarbonate safety shield.
Documentation
The device will be accompanied with a comprehensive instruction manual. The team
achieved this requirement by completing an instruction manual for the operation of the
device along with some safety recommendations. This manual is located in Appendix E.
Timing and Deadlines
All set deadlines and time requirements set out in the MECH 4010/4020 Design Project
Handbook will be met. Throughout the year, Team 12 has met or exceeded all deadlines
and requirements associated with the course work, deliverables, and testing.
Data Acquisition
The device should provide data describing the force, displacement, and time for each
trial. This design requirement was met and the output from the device is discussed in
detail in later sections. An LVDT and Bending Load Cell are incorporated in design and
the data is processed using DAQ.
3 Generation of Alternatives
The following section presents two alternative designs that were considered and the
benefits and the drawbacks of each design. The designs of this section were rejected with
the accepted design presented following this section.
3.1 Hopkinson Split Bar Apparatus
The Hopkinson Split Bar apparatus (HSBA) is used extensively in materials testing
because of its ability to achieve extremely high strain rates. The HSBA functions by
propelling a striker tube (using a compressed air gas gun actuator) towards an incident
bar. When the striker tube impacts the incident bar, a pulse wave is transmitted through
the incident bar and into the sample. Some of the pulse wave is then reflected back
through the incident bar (and captured in the momentum trap bar) and some is dispersed
in the transmission bar. Using an Enhanced Laser Velocity System (ELVS), the
deformation and velocity of the sample can be measured dynamically. This data would
then be input into The DAC card to be analyzed and recorded. The HSBA is shown in
Figure 5.
6
The main benefit of this design is that extremely high strain rates have been reported,
additionally, the ELVS can attain the dynamic stress strain curve. However, there are
several drawbacks of this design. Firstly, the gas gun raises several safe
safety
ty concerns, as
extremely high pressures are needed to propel the striker tube. Additionally, the HSBA is
difficult to calibrate, as pulse wave magnitude has to be finely tuned to the pressure the
gas gun actuator. Finally, we foresee this device being eexpensive
xpensive because the
compressed air system needed for the gas gun and, incident and transmission bar have to
be machined to very accurate specifications.
Figure 4: Hopkinson Split Bar Apparatus
3.2 Gravitational Impact Pendulum
A method of biological specimen testing biological specimens based on a gravitational
impact pendulum was considered. Figure 6 outlines the core components of such a
device. A pendulum-based
based approach has been employed in metallurgical testing to
determine properties such as hardness, necessary modifications would be needed to meet
the specifications set by the client. A pendulum
pendulum-based
based design would be capable of
meeting the objectives for lifetime, conditions, control and data acquisition as defined in
our Design Memorandum.
The pendulum-based
based design however was considered likely to fail on the objectives of
our chosen size and control criteria. A base size of 30in x 30in was deemed optimal for
the laboratory setting of the device, and it was likely that tthe
he swing of such a pendulum
would be capable of fitting within these dimensions while achieving the rate required. A
pendulum approach was considered also to be susceptible to control issues as a
mechanism for reproducibly determining required drop heights from requested strain
rates was needed.
7
Figure 5: Gravitational Impact Pendulum
The gripping system will consist of corrosion resistant material. A corrosion resistant
material is necessary because the biological specimen will be placed in a corrosive
aqueous saline solution. It was suggested by the client that Polyacetel would be a
preferable material due to its non-hydroscopic (not absorbing liquid)) characteristic.
characteristic If the
material is to absorb liquid, the assembly may not com
comee apart easily after long exposures
to the liquid. The grips will be mounted on a track to eliminate buckling and restrict the
strain to the axial direction.
4 Selected Design
The following section outlines our selected design and the mechanism that we used
use to
select it from the alternatives.
4.1 Design Evolution
The selected design consists of several different operational sections. The device is
broken down into the flywheel and motor, the engagement m
mechanism,, and the gripping
device sections.
The flywheel is a reasonable design due to its significant moment on inertia resulting in a
storage device for rotational energy. This characteristic lends itself to be a useful device
in controlling a constant velocity. The flywheel design was initially thought
hought to be
constructed out of stainless steel with an estimated diameter of twenty centimeters. The
brushless motor will have variable speed capabilities so the operator may test under a
range of speed variables and thus, strain rates. The engagement of the actuation method
will be performed in response to an electrical signal generated by the data acquisition
system.
8
Several methods for engaging the flywheel were under consideration. Initially, the design
was thought to hold a lightweight weight engagement wheel that will not continue to
rotate with the flywheel after the needed energy is removed from the flywheel with the
use of a sacrificial part, electromagnetic engagement, and a clutch system.
The gripping system consists of corrosion resistant material. A corrosion resistant
material is necessary because the biological specimen will be placed in a corrosive
aqueous saline solution. The grips will be mounted on a track to eliminate buckling and
restrict the strain to the axial direction.
Figure 6: Early depiction of LAHVTR
Breaking the design into several sub-components, and listing the various solutions to
these sub-components obtained from brainstorming sessions obtained the selected design.
Next, reasonable combinations of design sub-components were listed as design solutions
as shown in the morphological chart in Appendix B. Following this, the reasonable
combinations were then ranked based upon criteria that were deemed integral to the
design. From this ranking process, we were able to attain the three design ideas discussed
in this report. Finally, the three designed were assessed using the House of Quality as
shown in Appendix B.
The House of Design determined that the flywheel design satisfied the outlined criteria
the most. The team supervisor and client accepted the flywheel design and the team
began brainstorming more detailed design specifications.
The first design consisted of two flywheels and a clutch. One of the wheels held one side
of the sample and does not rotate. The second wheel is stationary until the motor is at full
velocity and the clutch is engaged to the flywheel, which breaks the specimen. This flaw
in this design lies in the fact that having two wheels is unnecessary if the gripping
mechanism is attached to the ground and the moment of inertia of the flywheel is not
being utilized.
9
The next design consisted of the gripping mechanism situated on the tabletop to enable
the filming of the specimen breaking with a high-speed camera. The flywheel was
connected to the motor and when the flywheel reaches the needed velocity an
electromagnetic clamp engaged the flywheel. This would pull a cable that is attached to
the gripping apparatus. This design was not chosen because a mechanism could not be
sourced that could grip the flywheel quickly enough to get up to the velocity needed to
break the specimen.
4.2 Design Selection Matrices
Breaking the design into several sub-components and listing the various solutions to these
sub-components obtained from brainstorming sessions resulted in design selection. Next,
reasonable combinations of design sub-components were listed as design solutions as
shown in the morphological chart in Appendix B. Following this, the reasonable
combinations were then ranked based upon criteria that were deemed integral to the
design. From this ranking process, we were able to attain the three design ideas
discussed in this report. Finally, the three designed were assessed using the House of
Quality as shown in Appendix B.
5 Problem Exploration
In order to more clearly understand the intricacies of the design and conceptualize the
requirements, the team completed research and testing on several aspects of the design.
Initial testing was conducted to determine the validity of the engagement mechanism and
solenoid operation. A mock-up of the device was constructed to determine the optimal
placing of members and a rapid prototyped model was created to confirm the final
dimensions of the device.
5.1 Modeling
Several methods of modeling were performed to aide in the conceptualization of the
device, determination of high stress areas, and the dimensions of the engagement
mechanism with respect to the flywheel.
5.1.1 LAHVTR Mock-Up
First a mock-up of the device was constructed to help the team conceptualize the aspects
of the design that required revision. The mock-up additionally enabled the group to
foresee the initial placement of the components around the flywheel. The model was
constructed from Foam core and wooden dowels.
10
Figure 7: Mock-up of Device
5.1.2 Finite Element Analysis
Preliminary finite element analysis was applied to the flywheel to find the range of stress
concentrations in the contact area of the flywheel. This initial analysis is represented as a
point load and depicted in Appendix C. The team discussed the utilization of modeling a
more precise finite element analysis model for the completion of the final design but the
concept was passed over due to lack of usefulness.
5.1.3 Rapid Prototyping
Representations of the engagement mechanism and flywheel were created using rapid
prototyping. These components were then precisely mounted on a wooden frame to
determine the validity of the design concept and finalize tthe
he dimensions of the apparatus.
Figure 8: Rapid Prototype of Device
11
5.2 Testing
Some rudimentary tissue testing was conducted prior to construction of any device to
aide in problem exploration.
5.2.1 Biological Specimen Testing
Initial testing was performed on the Loading Apparatus for High Velocity Tissue Rupture
to verify the successful operation of the engagement mechanism. Biological tissue was
loaded into the LAHVTR clamps and the pin was manually engaged through the circuit
board. Testing was performed with the flywheel at four speeds. Table 4 outlined the
information gained during the testing.
Table 4: Biological Specimen Testing Results
Test
1
2
3
4
Flywheel Speed
[rpm]
300
500
700
1000
Successful Engagement
Mechanism Employment
Yes
Yes
Yes
Yes
Observations
No wear
Engagement pin surface chip
Engagement pin surface chip
Engagement pin deformation
5.2.2 Solenoid Testing
The solenoid was tested under several applied voltages to determine the time encountered
when fully extending the solenoid core. This time period was then compared against the
necessary time period needed to successfully engage the pin with the flywheel. Five trials
were performed and the average of the data was recorded. Figure 9 compares the
experimental to the theoretical data of applied voltage to time encountered for full
extension of the solenoid core. The obtained data shows that the solenoid is well within
the needed operating range for the task of engaging the pin to the flywheel.
12
Time to Extension (ms)
Solenoid: Voltage - Displacement Relationship
70
60
50
40
30
20
10
0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
Applied Voltage (V)
Figure 9: Solenoid voltage vs Time (theoretical-red, experimental-blue)
6 Calculations
A valid concern about the flywheel driven impact apparatus was its ability to provide an
acceptable output displacement profile. An acceptable profile has constant strain rate past
the transient period and a transient period of no more than one-fourth the fracture strain1.
This document seeks to provide an assurance that an acceptable profile will be attained.
Figure 10 attempts to illustrate both an acceptable strain rate profile and the anticipated
profile which has been obtained from our calculations that follow.
Figure 10: Desired vs Anticipated Strain Rate
1
Meeting, 30th September 2009.
13
In this scenario the motor is disconnected from the inertial load (flywheel) through the
use of an overrunning clutch. To the end of obtaining the profile, the principal of inelastic
impulse and momentum is applied. The properties of the flywheel (w) and specimen grip
(g) are selected to be:
m w = 16 kg
m g = 1 kg
d w = 20 cm
I w = 0.061 kg ⋅ m 2
ω w,1 = 100 rad / s
Conservation of angular momentum (about point A, the center of flywheel rotation)
results in the expression:
( H A ) = ( H A )2
Iω1 = Iω f + m g v g r
(0 .061 kg ⋅ m 2 )(100 rad / s ) = ( 0.061 kg ⋅ m 2 )ω f + (0 .1 kg ⋅ m )V g , 2
(1)
The result is one equation with two unknown quantities. An additional bit of knowledge
is needed in this case. The coefficient of restitution, a function of the material in impact,
is applied. This coefficient relates the velocities of the masses along the line of impact,
just before and after the collision (Hibbeler, 2007). This coefficient, for steel, is e = 0.95
but is susceptible to both changes in geometry and surface hardening.
e=
e=
0.95 =
V g , 2 − Vw, 2
Vw,1 − V g ,1
V g , 2 − rω f
rω i
V g , 2 − rω f
(2)
(0.1 m )(100 rad / s )
Solving (1) and (2) results in the following:
• a final grip velocity of 16.75 m/s.
• a final grip kinetic energy of 140 J.
• a flywheel angular velocity of 72.5 rad/s.
• a final flywheel kinetic energy of 160 J.
• energy losses of 11 J.
These answers coincide strongly with solutions obtained through the application of the
“Working Model” software package with errors (in terms of energy) of less than 7%.
14
Friction and the load posed by the sample are thought to have no effect on the motor of
the grip. The following calculations highlight this reasoning.
For the load, assuming the specimen to be spring-like, we were told that the breaking
force was 15 N at a length of 3mm, so the spring constant is about (with a safety factor)
10000 N/m. This generalization is likely weak. The energy dissipated in a stretch to
breaking for the specimen should be about 0.045 J. These calculations follow below (with
an additional safety factor present for strain distance - ∆x is said to be the maximum
elongation of the specimen).
15 N
(SF ) = 10000N / m
0.003 m
1
1
2
2
= k (∆x ) = (10000 N / m)(SF ⋅ 0.003) = 0.5 J
2
2
k=
E lost − ( specimen )
(3)
(4)
The frictional losses are based upon the coulomb model for a µ k = 0 .3 which is common
for steel-on-steel surfaces without lubrication. The losses due to friction and (work
against gravity in the vertical configuration) are expected to be about 0.5 J and are
presented below.
E lost −( friction ) = µFn (∆x ) = 0.3 ⋅ (1 kg ⋅ 9.81 N / kg ⋅ SF ) ⋅ (0.003 m ⋅ SF ) = 0.2 J
(5)
Elost −( gravity) = mg (∆x ) = (1 kg ⋅ SF ) ⋅ (9.81 N / kg ) ⋅ (0.003 ⋅ SF ) = 0.3 J
(6)
The mass of the grip (1 kg) and its velocity at impact provides for more than adequate
amounts of kinetic energy to overcome friction, gravity and the specimen without
appreciable changes to its velocity.
The amount of time to reach the desired velocity following impact is determined from the
following expression where σ is the speed in sound within the material (for steel = 5000
m/s)2:
∆t = 2
L
(7)
σ
The time was found to be 6 µs for a pin of 2 inches in diameter. Since the pin is of less
than this diameter the value for this “ramp time” is surely less than any maximum
acceptable value.
Due to the variable nature of the coefficient of restitution it is only through
experimentation following the build of a prototype that a solid relationship between the
2
http://tinyurl.com/yea2xt4
15
flywheel rate of rotation and the linear velocity of the specimen grip may be determined.
However these calculations show:
•
•
•
that the desired velocity (and thus strain rate) will be obtained in the transient
region.
that the resulting displacement profile should be free of large decelerations in the
time leading to fracture.
and that a solid relationship between ω and e exists through an experimental
determined quantity e.
Calculations were performed regarding the torque that the shaft was subjected to. With
these results, the team designed the method of connecting the flywheel to the shaft. The
team plans to improve on the gripping mechanism performance by researching methods
of decreasing the mass of grip that undergoes linear motion upon impact.
7 Final Design
The second term of this design project allowed for refinement of the original design and
the implementation of the measurement systems. The final design is presented in Figure
11. This section will detail the system in terms of both mechanical components and
measurement/control systems.
Figure 11: Final Design
7.1 Drive Shaft Assembly
The drive shaft (Figure 12) assembly of this system consists of a flywheel which is driven
by a 3 phase motor (more specs) under frequency control. The drive shaft is coupled to
the flywheel shaft by means of a flexible spider coupling.
16
Figure 12: Drive shaft Assembly
7.1.1 Flywheel Design
When designing the flywheel the mass, size, and the cost of the flywheel were the main
considerations. Increasing the mass, increases the moment of inertia that increases the
available energy that is available for transfer to the specimen during impact. The design
utilizes this energy (and the subsequent force) to accelerate the gripping mechanism that
fractures the specimen.
The flywheel is machined out of 0.2 m diameter carbon steel. This material was chosen
for it’s low cost and high weight. It is also a common material that can be ordered locally
which will reduce the cost further. The flywheel was modeled after a disc of uniform
thickness. The failure calculations show the flywheel will not fail at the intended angular
velocity of the shaft. These calculations are located in Appendix C.
The flywheel was designed with flanges to decrease stress concentrations of the flywheel
at the critical areas for shear stress. Tresca’s theory of failure was used to find the
maximum shear stress. The flanges are not included in the failure calculations of the
flywheel though they add a factor of safety to the design. The diameter of the flywheel is
sufficient such that, the arc of motion needed to fracture the tissue can be modeled as
approximately linear.
17
Figure 13: Flywheel
Figure 14 is a plot of the flywheel diameter (series2, constant thickness of 5cm) and
thickness (series1, constant diameter of 20cm) versus the flywheel mass. The plot used a
density of carbon steel for calculations. This graph was created to give Team 12 an
understating how the flywheel mass changed with changing dimensions. This was a
valuable tool that helped select the flywheel dimensions as the team could quickly see the
effect of changing
ng parameters.
Figure 14: Flywheel Design
The flywheel is fitted with a pin (Figure 15) that contacts the actuating pin on the
gripping mechanism. This pin and actuating pin are constructed from carbon steel. This
material iss hard and strong and holds low ductility. These characteristics are necessary for
the high impact loading presented to these items.
18
Figure 15: Engagement Pin in Flywheel
7.1.2 Main Shaft
The main shaft to which the flywheel will be mounted is constructed of carbon steel; it is
to be one inch in diameter and eight inches long. The choice to construct the shaft of
carbon steel was a budgetary decision
decision. The shaft was a donation from the Mechanical
Mech
Engineering Department.
Furthermore, constructing the shaft with a one
one-inch
inch diameter was deemed ideal because
the static shaft deflection from the flywheel was calculated to be negligible at this
dimension.. Additionally, the critical shaft speed was calculated to be approximately 12
times the operating speed of 955 rpm. Figure 16 illustrates an exploded view of the drive
shaft assembly.
Lastly, Team 12 is considering different methods to couple the motor to the main shaft.
The team chose a flexible
le coupling due to its vibration management capabilities and ease
of alignment.
Figure 16
16: Exploded View of Drive Shaft Assembly
Shaft [1], Bearing [2], Flywheel [3], Coupling [4],
Spider coupling joint [5],and Motor [6]
19
7.1.3 Gripping Mechanism
The gripping mechanism consists of two grips that hold the sample at each end, a linear
sliding track, and support housing for bearings and the contact tooth. The grips are
machined out of stainless steel. One side will be attached to the force transducer while the
second end is attached to housing, allowing it to move down the sliding track. The
housing is machined from a stainless steel.. The contact tooth will be designed
designe using the
same carbon steel as the flywheel pin. Figure 17depicts
depicts an exploded view of the grip
assembly.
Figure 17: Exploded Grip Assembly
Grip [1], Grip top [2], Specimen [3], Engagement pin [4],
Tooth holder [ 5], Tooth housing [6], Guide rod [7], Bath [8]
7.1.4 Damper
The team estimated the forces involved in the operation of the engagement mechanism. A
pre made damper was sized using these calculations and purchased. This damper was
fastened on the end of the track to deter the tooth housing from rebounding and distorting
the sample.
20
Figure 18: Damper set-up
Engagement mechanism support [1], Force transducer [2], Grip [3], Specimen [4], Tooth
holder [5], Tooth housing [6], Grip top [7], Bath [8], Engagement pin [9], Guide rod
[10], Damping system[11], Damper [12], Mounting nut[13]
7.1.5 Safety Shield
Due to high rotational speeds and large impact forces, the device is enclosed in a safety
shield. The safety shield is to be constructed out polycarbonate
nate panels to aid in the
ability to view the flywheel operation
operation. The safety shield is depicted in Figure 11.
Figure 19 - Diagram of Safety Shield
7.2 Electrical Components
The electrical components of this project consisted of a motor and frequency drive, the
measurement systems, DAQ system, and high
high-speed
speed video camera equipment.
7.2.1 Motor and Frequency Drive
21
The device is driven using a 1/3 horsepower, 115-volt, three phase,
AC motor; controlled using an AC Drive. The motor was selected on
its ability to drive the flywheel at 1000rpm and operate using a
standard 115-volt power supply. An AC motor was selected over a
DC motor based on cost and ease of power supply.
Additionally, the frequency drive was selected based on its ability to
easily and accurately control the motor frequency (speed).
Furthermore, the drive is compatible with a PC computer, thus if the
client prefers, the motor can be controlled from a PC.
Figure 20: Frequency Controller
7.2.2 Measurement Systems
The measurement systems are all controlled through the DAC card. The operator initiates
the rotary encoder through the PC. The rotary encoder senses the position of the flywheel
and the solenoid is automatically deployed and the data requisition system is initiated to
collect the data.
7.2.2.1 Rotary Shaft Encoder
The pin is engaged (and consequently the specimen is loaded) both after the flywheel
reaches the desired speed and after the pin is in the precise location. Inability to locate the
pin could result in partial contact of the impacting surfaces and failed experiment and
destroyed specimen. A rotary encoder is mounted on the shaft and acts as a signal to
engage the solenoid.
The encoder is an infrared sensor that senses light passing between two sensors on either
side of the encoder. Only one hole is necessary on the rotary encoder. The computer
driver for the tissue rupture device was developed to allow for the pin to fire after
activation of the ‘fire’ button and completion of the revolution of the flywheel. This
allows for a failsafe method, ensuring the pin will never miss fire.
7.2.2.2 Stroboscope and Frequency Controller
The angular velocity of the flywheel determines the strain-rate experienced by the
specimen. The strain-rate is a controlled quantity thus measurement of angular velocity is
required. The Strobotac is used to determine the angular velocity of the flywheel in
operation by adjusting the frequency controller until the white stripe painted on the
flywheel appears to be in a constant position. The frequency controller displays the speed
of the flywheel in RPMs while the shaft is rotating.
22
7.2.2.3 Force Transducer
A standard strain gauge will be mounted to a titanium component on the device. This
configuration will permit measurement of force loading on the specimen. This is
illustrated in Figure 21. Calibration of the force transducer is required. The team
measured the voltage change as a result of the application of several calibration weights.
These output points were then used to determine a linear relation between the deflection
and voltage change. This relation is used to determine the force applied to the specimen.
The results of the calibration are discussed in the calibrations section of this report.
Figure 21: Specimen Force Measurement
The force cell was selected based on several criterions. Firstly, the strain gauges were
selected based on the theorized forces and other characteristics that would ease mounting
the gauges. The size and material of the load cell was determined. Titanium was
selected for its relatively low Young’s modulus (approximately half that of steel) because
the project forces were thought to be approximately several hundred Newton’s (based on
information obtained from the client). Furthermore, the nominal length and width were
fixed as a result of geometry. Therefore, working gauge length and cell width was used
to create a scenario that would result in strains that were compatible with the selected
gauges. The working gauge length was selected to be five centimeters; this was mainly
chosen for practicality reasons. Lastly, the thickness was based on available titanium
sizes, several thicknesses were considered, however, 2 millimeters yielded the best results
based on the equation below:
Where P is the force, x is the working gauge length, E is Young’s modulus and t is the
cell thickness.
Therefore, bases on the numbers described above, the strain was calculated to be 1700µε
which was well within the selected gauge specifications.
23
In terms of actual data measurement, the voltages were collected using a Wheatstone
bridge and amplified using a load cell amplifier. Similar to the other measurement
devices, the data was input into a PC via a DAQ card and processed using software.
The two gauges were placed into a Wheatstone bridge configuration as per Figure 22
with Rx = Ra = 350Ω and Rm = Rm = 350 Ω strain gauges.
Figure 22: Wheatstone Bridge Configuration
7.2.2.4 Linear Variable Displacement Transformer
As the core of the Linear Variable Displacement Transformer (LVDT) moves, the output
voltage increases from zero to a maximum. The magnitude of the output voltage is
proportional to the distance moved by the core. This output voltage is collected by the
DAC card, converted to distance (using calibration curves), and displayed in a graph of
voltage vs. time and distance vs. time.
7.2.3 Solenoid Actuation
The actuation of the contact tooth was conducted using a solenoid. The solenoid is
controlled through The DAQ card to enable precise deployment. Figure 13 depicts the
contact tooth in the initial position. The contact tooth will remain in this position until the
flywheel reaches the appropriate angular velocity. At that point, the technician will
activate The DAQ card and the solenoid will push the contact tooth into the final
position. In this position the contact tooth will meet the flywheel pin. The flywheel pin
will move the contact tooth, along with the grip, along the track. This motion will rupture
the tissue. The second position is also depicted in Figure 24 and Error! Reference
source not found..
24
Figure 23: Initial Position of Contact Tooth
Solenoid [1],Contact tooth [2], Tooth holder [3],
Tooth housing [4], Pin [5], Flywheel [6]
Figure 24: Final Position of Contact Tooth
Solenoid [1]
[1], Contact tooth [2], Tooth holder [3],
Tooth housing [4], Pin [5], Flywheel [6]
25
8 Implementation of Measurement Systems
8.1 Calibration of LVDT
Figure 25, below shows a calibration curve for the LVDT. This was generated by
measuring the voltage at known distances from the starting position. The data was then
correlated using a linear regression to project the measured voltage to a given distance. If
one notes the R2 value of 0.9982, the data is almost a perfect fit.
LVDT Calibration
7
y = 6.1484x - 0.9763
R² = 0.9982
6
Displacement (mm)
5
4
3
2
1
0
-1
0
0.5
Voltage (V)
1
1.5
Figure 25: Calibration Graph for LVDT
8.2 Calibration of Force Transducer
The calibration of the force transducer was done in a similar fashion to the LVDT.
Known masses were hung from the load cell and voltages were recorded. The masses
were then multiplied by the acceleration due to gravity to get the force applied. Again
the data was fit using a linear regression and is shown in Figure 26. Once again note the
nearly perfect linear match with an R2 value of 0.9991.
26
Force Transducer Calibration
35
30
y = 12.371x + 0.6334
R² = 0.9991
25
Voltage (V)
20
15
10
5
0
0
0.5
1
Force (N)
1.5
2
2.5
Figure 26: Calibration Graph for Force Transducer
9 Safety
During the design process creating a safe device was of the utmost importance.
Therefore, we considered three levels of safe design, including; procedural safety,
engineered safety and inherent safety.
Firstly, for procedural safety, Team 12 wrote an operation manual. This operation
manual outlines safe operation of the device as well as clearly outlining the risks and
hazards associated with device operation. Secondly, we encased the device in a safety
shield constructed of polycarbonate panels. This will ensure the operator is safe should
the device catastrophically fail. Lastly, the design we have created is inherently safe
because the operator can control the device remotely from a computer, away from the
actual device. Safety precautions are outlined in the User’s Manual in Appendix D.
10 Testing
The following sections outline our testing procedure, our methods of validation, and the
data that we were able to collect.
10.1 Test Procedure
1. The specimen is sized and placed in specimen grips.
2. The specimen is placed in pretension by adjustment of the force transducer. The
Allen key tightens the setscrews at the desired tension.
3. The zero position of the LVDT in the centre of the throw so centre the initial
position around the centre of the entire throw to obtain the most accurate results.
27
4.
Place the safety case over the flywheel section.
5. Turn on the Stroboscope and set the RPM dials to the desired angular speed.
6. Turn on the frequency controller and gradually increase the speed until the white
stripe on the flywheel appears to be stationary. This indicates that the angular
speed of the flywheel is the same as the displayed angular speed on the
stroboscope.
7. Press enter to record the data.
10.2 Validation Method
The LVDT and the force transducer were calibrated to aide in verifying the method used.
While the machine is stationary, the voltage output for several distances was rerecorded
and these points were used to create a calibration curve. The device is activated and the
output data from the test is compared to the calibration curve to verify the validity of the
data.
Furthermore, the LVDT was validated using a high speed camera and video processing
software. Testing was recorded using video capture at 500 frames per second. The
acquired video was then processed and the software was able to match image points in
several frames over time and output velocities and positions.
Additional validation of the load cell was considered. This potentially could be
completed using the video processing software. On the video one can track how the load
cell moves linearly when the sample is stretched. Using this information in addition to
the properties of the cantilever load cell (length, height, width, material) and can
calculated the required force to move the load cell the distance calculated in the video
software. Given the high R2 value of the load cell, this additional validation was deemed
unnecessary.
10.3 Measurement Equipment Testing Results
When tested the signals of Figure 27 were acquired.
28
Figure 27: Raw Signals Acquired During Testing
The signals were processed and are presented in processed form in Figure 28. The rootmean-square
square of the LVDT signal was taken for each half
half-period
period and then multiplied by
because Vpeak = Vrms. Label “A” of the figure indicates the action of the gas spring.
Label “B” indicates a region of constant velocity over a long distance of travel. Label “C”
indicates that the specimen displaced before it strained and that the sample was likely
under-tensioned. Label “D” indicates resonance of the beam which was easily seen under
u
high-speed video analysis.
Figure 28: Processed Signals
29
When our signals were analyzed on the basis of velocity vs. time we noticed the trends
seen in Figure 29. This figure shows that our transient period, where acceleration is
occurring, is about 8mm for the case of 1000 RPM. This transient period is much greater
than the period we required of approximately 1mm. A region of near-constant velocity
was achieved. It would be preferable to engage and break the specimen in this region as
the region is of sufficient length to break the sample.
Figure 29: Velocity vs. Time Analysis
When analyzing both force and displacement simultaneously we expected to see the
results of Figure 30. Our bovine pericardium samples are hybrid tissues with light strain
characteristics dominated by elastin and high-strain characteristics dominated by
collagen.
Figure 30: Documented Force vs. Displacement Characteristic for Sample
30
We were able to replicate this force-displacement curve using our device. The trends we
witnessed are presented in Figure 31. We also were able to see a strain dependence of the
resisting force. This data would not be of large significance to BME researchers because
the data was collected during the acceleration phase of our device and we did not achieve
the targeted 300s-1 rate when breaking the sample.
Figure 31: Force-Displacement Characteristic Obtained
10.4 High Speed Video Testing Results
In order to calculate the strain rate achieved during testing the approach was taken to use
a high speed video camera and analyze the video using Photron Motion Tools software.
Several videos were taken and analyzed; however, the one below was taken at 500 frames
per second and a flywheel speed of 500RPM.
The selected software allowed the user to input a distance frame of reference, which then
converted the distance to pixels per unit. Shown in Figure 32, is the user putting the
distance of a bolt head of five millimeters, then software then converted the scale to 6.4
pixels per millimeter.
31
Figure 32: Creating a reference system for high-speed video analysis.
In the next step the user picks a location on the moving part and tracks it though several
frames of video. The four frames below illustrate this process. In frame [1] a point on the
moving part to be tracked is selected and tracked in subsequent frames (shown by red and
green tracking dots). All of the frames are time stamped which allows for accurate data
analysis.
Figure 33: High-speed video frames and feature tracking for 8mm
sample at 500RPM and 500 frames per second.
After the points have been selected the data is output to Excel for analysis. The data
acquired from this trial is shown in
Table 5.
Table 5: Displacement data for 500RPM trial with 8mm sample
Frame
Time (seconds)
Track Point 1 (mm)
32
x
-2649
-2648
-2647
-2646
y
-5.2980003
33.125
28.125
-5.2960005 35.15625
28.125
-5.2940001 46.09375
28.125
-5.2920003 57.1875 27.96875
From the output data, the velocity was found by dividing the distance by the time. In this
case the maximum velocity was found by analyzing frames 2646 and 2647.
Δ
Δ
57.1875 46.09375
5.2920003 5.2940001
5547 /
From the calculated velocity and know the sample gauge length (8mm in this case) the
strain rate can be found using the following formula:
5547
693.375 s!"
8
The maximum strain rate found for the device was calculated to be 814 s-1; this was
calculated using the same method as above at 1000RPM and an 8mm sample. The data
acquired is outlined below in Table 6. Furthermore, it should be noted that the velocity
below was in fact very similar to the LVDT velocity acquired for testing the device at
1000RPM; however, it was less than the predicted 10m/s during the design phase.
Table 6: Displacement data for 1000RPM trial with 8mm sample
Frame Time
-7218
-7217
-7216
-7215
-7.218000412
-7.217000484
-7.21600008
-7.215000153
Track Point 1
x
y
15.3488369 22.5581398
17.4418602 22.5581398
21.9767437 22.5581398
28.4883728 22.6744194
For this set of data the team looked at frames 7215 and 7216.
Δ
Δ
28.4883728 21.9767437#
7.215000153 7.21600008
33
6512 /
6512
814 s !"
8
It fell short of our design requirement of 1000s-1 however; we are quite satisfied with the
result. Some of the items that held the team back in achieving the design requirement,
included plastic yielding of the engagement pin at 1000RPM, and as a result the team
could only effectively test the device up 700RPM on a regular basis. Furthermore, the
device did not show the expected behavior and higher speeds did not always yield the
best results. Additionally, the team did not see the acceleration that was expected
(partially due to plastic yielding of the pin) and thus strain rates were not as high as 1000
s-1 as the sample was breaking during the acceleration and not at the maximum velocity.
10.5 Time of Impact Analysis
The bending force on the pin can be used to determine the interval of contact. The
following calculations illustrate this phenomenon.
$%&$ 205 ' 10( )
$*$ +&
,
$+$ -
0.0.00692#1
(
.205
'
10
)#
/
2
64
$- $ *, 508.32 3
&
0.016.0.00346#
$$ 456789, ;78, 9<7;, =5<> 1 .) 5? >) )8 9<7;#
3
1
$$ 2399 .13.2069# 243.49
3
- 508.323
) 2088.4/ 0.243@9
- 508.323
) 2088.4/ 0.243@9
6/
2.873 ' 10A B 3
) 2088.4/ Our design requirements stated an incidence time of 1ms. However were not able to
theoretically derive the time interval without knowing the forces that would be
encountered.
34
11 Issues Encountered
This section outlines some of the issues that were encountered in testing and construction.
11.1 Electrical and Mechanical Crosstalk
Some of the unexpected results obtain from our data capture may have been a result of
crosstalk.
Crosstalk was likely present in the propagation and coupling to the field, and between
wires in our electronic system. The most common form of this crosstalk between
electrical lines is where a transmission line shares a ccommon
ommon path with another line
(Christopoulos, 2007). In this case, current flow on one line results in an interference
signal on the other line. Radiated interference is another coupling path. This interference
does not involve a physical connection betwe
between the circuits
Another form of crosstalk we may have observed is that of mechanical cross-coupling
cross
depicted in Figure 34.. The moving components (the housing, pin, aand
nd gripper) (shown as
m2) are coupled by the tissue (k2, c1) to the force transducer and grip (k1, m1). This was
problematic for us as our measurement equipment was connected to the two cross
coupled systems.
Figure 34: Mechanical Cross-Coupling
12 Impact on Society
In creating an effective and reliable device we hope to give Dr. Michael Lee a research
tool that will help progress the state of biomaterials research. Thus, with this research
35
tool, valuable information will be obtained in how biomaterials fail in high impact (high
strain rate) settings. With this information, Doctors could possibly learn how to better
treat patients that have suffered impact injuries.
13 Life Cycle Analysis
The specimen’s linear displacement and velocity will be fully determined when testing
begins. At this point, the team will complete the design of the data acquisition system,
specifically, regarding the LVDT measurement device and software.
14 Work Allotment
All fabrication was completed by in the Mechanical Engineering Machine Shop. The
design entails cutting and machining. Table 7 compares the estimated time of machining
to the actual amount of machining time required.
Table 7: Comparative Chart of Estimated to Actual Machining Hours
Part Description
Quantity
Flywheel
1
Flywheel pin
3
Grip top
2
Grip bottom
2
Tooth housing
1
Contact tooth
2
Frame
1
Plexiglas
1
Tooth holder
0
LVDT support
0
Force Transducer
0
Total Hours of Machining
Estimated
Hours
20
8
6
6
10
5
10
5
0
0
0
70 hrs
Quantity
1
1
2
2
1
10
1
1
1
1
1
Actual
Hours
18
3
5
5
11
10
10
4
7
3
4
80 hrs
The estimation of hours was low because some of the material for the parts was changed.
The parts that were changed from carbon steel to stainless steel were more difficult to
machine. The Mechanical Engineering Technicians completed the machining and the
team completed the assembly, painting and electronics.
15 Budget
The estimated and final budget is illustrated in Table 4 and a detailed budget is located in
Appendix D.
Table 8: Comparative Graph of Estimated to Actual Budget
Mechanical
Estimated
Actual
36
Frame
Flywheel
Specimen grip
Shock Absorber
Bearings
Main shaft
Electrical
Motor
Measurement
Tooth Engagement
Control
Power
Function Generator
Safety
Polycarbonate
Modeling
Rapid Prototyping
Misc
$150
$40
$315
$0
$0
$65
$105
$40
$172
$67
$30
$0
$335
$1000
$16
$330
$30
$0
$335
$660
$16
$335
$0
$375
$0
$130
$0
$234
$1220
$50
$3500
$2549
Total
The project was funded both by the Department of Mechanical Engineering and the
Department of Biomedical Engineering.
16 Future Considerations
This section outlines some refinements to the device that could be made in the future.
16.1 Control of Initial Sample Length.
The client has expressed that the ability to load samples into the apparatus to a
predetermined length is desirable. At this point the operator loads the sample, tensions it,
and then confirms the length. The current method of tensioning the sample is by moving
the mounting block along the line of the arrow shown in Figure 35.
37
Figure 35: Current control of sample length
In the future, a calibrated drive screw mechanism could be implemented to move the
mounting block with precision. This design would be similar to what is found on a
precision microscopy stage.
16.2 Prevent deformation of critical components
In an effort to reduce deformation of the tooth component, the mass of the moving parts
could potentially be reduced. This could be done by reducing mass by part redesign.
Furthermore, material could be removed by drilling holes in non-critical areas of the
moving parts. Other suggestions included changing material to aluminum or titanium.
However, other issues arise with aluminum such as corrosion (due to the saline solution),
and low yield strength. As well, titanium is substantially more expensive than steel, and
more difficult to machine.
One example of reducing mass by component re-design would be to move the location of
the spring acting on the pin. The current design shown in Figure 37 incorporates the
spring below the tooth. Mass could be removed by placing the spring above the tooth
holder.
38
Figure 36: Cross-sectional view of spring loaded tooth.
Contact Tooth [1], Tooth holder [2], Tooth housing [3]
16.3 Other Considerations
The pin could be hardened to withstand tests with the motor run at 1000rpm. The shaft
could be stepped to reduce ware when assembling/disassembling. In the future, a stainless
steel bath could replace the current plastic one. This would allow the client to steam
sterilize it between uses. The moment acting on the tooth could be reduced by
minimizing the vertical distance between the bath, and the outer rim of the flywheel. This
could be achieved by shimming the bearings, and motor mounts. Also, in an effort to
reduce electrical crosstalk, dedicated circuit boards could be created, and shielded.
17 Conclusions
In conclusion the device constructed is able to achieve a strain rates of 800s-1 for
samples of a typical length. This figure is 200s-1 short of our intended goal of 1000s-1. In
addition a greater than anticipated transient period of acceleration was encountered
indicating that minor machine modification would be necessary before the device is
capable of research-quality work.
Despite these shortfalls many successes were encountered.
39
18 References
Cheng, M., Chen, W. Weerasooriya, T. “Mechanical Behavior of Bovine Tendon with
Stress-Softening and Loading rate Effects.” Advanced Theory of Applied Mechanics V2,
n2, 59-74. (2009)
Chen, W., Lu, F., Zhou, B. “A Quartz-crystal embedded Split Hopkinson Pressure Bar
for Soft Materials.” Experimental Mechanics. V40, n1, 1-6 (March 2000)
Willett, T., Labow, R., Avery, N., Lee, M. “Increased Proteolysis of Collagen in an In
Vitro Tensile Overload Tendon Model.” Annals of Biomedical Engineering.V35, n11,
1961-1972. (2007)
Active Power. Understanding Flywheel Energy Storage: Does high speed really imply a
better design? White paper 112. <www.activepower.com>
Callister, W. Materials Science and Engineering, An Introduction, Edition Seven. John
Wiley and Sons, Inc. (2007)
Lee. M. (2010) MECH 4650 Course Notes.
19 Appendices
List of Appendices
Appendix A – Previously Signed Documents
Appendix B –Decision-Making Tables and Charts
Appendix C –Gantt Chart
Appendix D –Budget
Appendix E –User's Manual
Appendix F –Engineering Drawings
Appendix A– Previously Signed Documents
List of Documents
Project Agreement
Project Waiver
Memorandum
40
Design Project Agreement
This Agreement is made and entered into for a term beginning the ____day of September 2009,
and ending the ____day of May 2009 between:
Ben Breen
Ruth Domaratzki
Geoff Beck
Rachael Schwartz
(hereinafter referred to as “Design Team”)
And
Dr. Michael Lee
(hereinafter referred to as “Client”)
The Design Team and Client hereby agree as follows:
1] Scope of work
The scope of work agreed upon within the memorandum.
2] Principal investigators
The Principal investigators of the design project shall be the Design Team as stated above.
(Students at the Department of Mechanical Engineering, Dalhousie University)
3] Confidentiality and Publication
The project and corresponding presentations, reports and web pages will be in the public domain.
4] Ownership of Intellectual Property
The intellectual property will remain the shared property of the Client and Design Team. The
fabricated device will be the property of the Client. The project presentation and reports (with the
exception of some possible proprietary information) will be in the public domain.
41
Design Project Waiver
This Agreement is made and entered into for a term beginning the ____day of September 2009,
and ending the ____day of May 2009 between:
Ben Breen
Ruth Domaratzki
Geoff Beck
Rachael Schwartz
(hereinafter referred to as “Design Team”)
And
Dr. Michael Lee
(hereinafter referred to as “Client”)
The Design Team and Client hereby agree as follows:
Indemnity
Each party shall indemnify and save harmless the other party against all costs, actions, suits,
claims, losses, or damages for all matters arising out of this agreement and the performance of the
project. The Client shall indemnify the Design Team against all costs, suits, or claims resulting from
the use by client or licensees of and deliverables or intellectual property developed by the Design
Team under this agreement.
Warranties
Dalhousie University and the Design Team shall not be liable for any direct, indirect, consequential,
or other damages suffered by the Client or any others resulting from the project or the use of the
research results and data from the project or any such invention or product.
Entire Agreement
This agreement constitutes the entire agreement between the parties with respect to the subject
matter hereof and supersedes all prior agreement, whether written or oral.
42
Memorandum
To: Dr. Julio Militzer
CC: Dr. Michael Lee, Dr. Kujath
From: Group 12 – Ben Breen, Ruth Domaratzki, Geoff Beck, and Rachael Schwartz
Date: 03/10/09
Re: Design Requirements for Loading Apparatus for High Velocity Tissue Rupture (LAHVTR)
Summary of the Project
Our group will be designing a loading apparatus with the capability to conduct fractures of a controlled and
rapid nature in collagen-rich tissues (bovine tendon) for Dr. Lee of the school of Biomedical Engineering.
After meetings with Dr. Lee, the design requirements have been set, and are outlined below.
Design Requirements
The requirements have been set under the following categories:
Size: The device should be able to fit on a table top with one face approximately 30 ×30 cm .
Strain Rate: The strain rate achievable should be on the order of 1000
Loading: Achieve a minimum of 1/100 loading
Lifetime: Should last approximately 5 yrs
Conditions: The conditions of the test sample will be as close as possible to physiological conditions (100%
humidity at 37˚C)
Control: The device will be designed so the operator has control of the strain rate.
Safety Features: The device will be designed to be safely operated by trained individuals. A shielding
component will be incorporated if required.
Documentation: The device will be accompanied with a comprehensive instruction manual.
Timing and Deadlines: All set deadlines and time requirements set out in the MECH
4010/4020 Design Project Handbook will be met.
Data Acquisition: The device should provide data describing the force, displacement, and time for each
trial.
Intellectual Property
The intellectual property will remain the shared property of the Client and Design Team. The fabricated
device will be the property of the Client. The project presentation and reports (with the exception of some
possible proprietary information) will be in the public domain.
Provisions by Client
The client will provide:
Time-weekly meetings
Funding- Fabrication costs, supplies
43
Appendix B: Decision-Making Tables and Charts
Group 12: Brainstorming Session
Morphological Chart for High Velocity Loading Apparatus
Table 1: Components of the Design Compared
1
2
3
Component
Propulsion
Velocity
measurements
Force
measurement
Pendulum
LVDT
HSBA
ADC + LPT1
Laser
Load cell
Solutions
DC Actuator
Piezo-based
Electromagnetic
Flywheel
Laser
Oscilloscope
Table 2: Options Selected
Design
1
2
3
4
5
6
7
8
9
Propulsion
Pendulum
Pendulum
Hopkinson bar
Flywheel
Flywheel
Hopkinson bar
Flywheel
DC Actuator
Velocity measurements
Laser
LVDT
Laser
Laser
LVDT
Laser
Laser
Laser
Laser
Electromagnetic
Force measurement
Laser
Load cell
Load cell
Load cell
Load cell
Laser
Laser
Laser
Load cell
Table 3: Evaluation Criteria
Complexity
1
2
3
4
5
6
7
8
9
9
10
2
9
6
5
8
8
4
Speed
Performance
7
7
6
10
9
10
10
4
6
Accuracy
in Speed
9
9
9
10
9
9
10
4
7
Controllability Accuracy in
in Speed
Measurement
6
7
6
8
8
10
9
9
9
7
9
10
9
9
4
6
7
8
Cost
Estimate
7
9
7
10
7
5
1
7
3
Total
45
49
42
57
47
48
47
33
35
Table 4: Tabulation of Votes
Design
6
2
4
Ruth
3
2
1
Rachael
1
3
2
Geoff
2
3
1
Ben
3
2
1
Total
9
10
5
44
House of Quality
45
Appendix C: Gantt Chart
(see attached)
46
Appendix D: Budget
Item Description
0.06" Sheet Metal
Rapid Prototyping
Stainless Steel
Shock Absorber
Flywheel
1.25" Square Tube
External retaining rings
Shaft
Plummer Block
Bearings
LVDT
Location in Design Hierarchy
Mech/Frame
Misc
Mech/Shaft
Mech/Frame
Mech/Shaft
Mech/Specimen Grip
Mech/Specimen Grip
Mech/Shaft
Item Status
Essential
Essential
Essential
Essential
Essential
Essential
Essential
Essential
Motor
Elect/Motor
Essential
Frequency controller
12VDC Pin Driving
Solenoid
Strain Gages
Precision Resistors
Titanium
Elect/Motor
Essential
Elect/PinEngagement
Elect/Measurement/Displacement
Elect/Measurement/Displacement
Elect/Measurement/Displacement
Essential
Essential
Essential
Essential
Polycarbonate
Function Generator
Miscellaneous
Safety
Elect/Control
Misc
Essential
Essential
Essential
Mech/Shaft
Essential
Elect/Measurement/Displacement Essential
Distributer
Metals-R-Us (Dartmouth)
Dalhousie
Metals-R-Us (Dartmouth)
McMaster-Carr
Metals-R-Us (Dartmouth)
Metals-R-Us (Dartmouth)
Mechanical Dept
Mechanical Dept
1
1
1
2
4
1
Total (No
Tax)
$74.88
$233.72
$172.39
$67.16
$40.00
$35.56
$0.00
$0.00
2
1
$30.00
$401.00
1
$226.78
1
$334.96
10
4
1
$15.27
$135.00
$80.00
$43.70
1
2
1
$130.71
$375.00
$50.00
Quantity
12
Fastenal
A-Tech
Motion Industries
(Dartmouth)
Motion Industries
(Dartmouth)
McMaster-Carr
Intertechnology
Intertechnology
Alfa Aesar
Piedmont Plastics
(Dartmouth)
Allied Electronics
TOTAL
$2,446.13
47
Mechanical Department Budget
Item Description
RP Prototyping
RP Prototyping
McMaster Carr
Metals R Us
Metals R Us
Piedmont Plastics
RP Prototyping
Ben Breen
RP Prototyping
RP Prototyping
RP Prototyping
Mechanical Department Funds
SELF Funds
Total
$106.86
$106.86
$55.80
$55.80
$131.84 $131.84
$25.64
$110.24 $135.88
$62.15
$62.15
$130.71 $130.71
$11.70
$11.70
$40.47
$40.47
$15.72
$15.72
$2.34
$2.34
$15.66
$15.66
TOTAL
$200.00
$509.13
GRAND TOTAL
$709.13
48
Appendix E: User’s Manual
User Manual for Loading Apparatus for High Velocity
Tissue Rupture
Team 12
Ben Breen
Geoff Beck
Rachael Schwartz
Ruth Domaratzki
This manual contains an overview of the safe operation of the Loading Apparatus for High
Velocity Tissue Rupture (LAHVTR)
49
1. General Use
The apparatus is designed to perform tensile tests on biological tissues at strain rates at a strain
rate of 1000s-1. The loading apparatus will fracture a specimen of bovine tendon (up to 2.5cm),
recording force, position, and velocity.
2. Safety Precautions
The Loading Apparatus for High Speed Tissue Rupture holds several areas of safety concern.
This section is to inform the operator of the risks and necessary precautions taken to operate the
equipment.
The flywheel is rotating at high speeds. The following precautions should be taken when
operation the device:
efore operating the motor. Refer to
Place the safety shield over the flywheel whenever bbefore
Figure 1 for the correct orientation of the safety shield.
Safety glasses and hearing protection must be worn.
Remove loose clothing and jewelry prior to operating the equipment.
Before each test, inspect the apparatus to make sure that no tools or wires are in the
vicinity of the flywheel.
Avoid contact with the shaft coupling
coupling.. The coupling experiences high temperatures during and
after operation.
Figure 1: Correct Safety Shield Orientat
Orientation
50
3. Initial Operation and start up checklist
Before operating the LAHVTR apparatus, read Section 2 of this manual carefully and adhere to
all safety precautions. Before each test session, complete a visual inspection of the apparatus and
complete the Start-up
up Checklist located in Appendix A.
4. Operating Instructions
4.1 The Strobotac
The Strobotac is used to determine the frequency of the rotation of the flywheel in operation. The
Strobotac is placed on the tabletop at a distance of one foot from the face of the flywheel.
Dimming the laboratory lights may aid in the ease of reading the frequency of the rotation of the
flywheel. The RPM dial is used to select the RPM range in which the operator will test. The
LAHVTR operates at a maximum of 100
1000
0 rotations per minute. For this reason, the operator will
select in the operating range of 110
110-690 rpm or 670-4170
4170 rpm. The outer surface of the RPM dial
is the precision control. The dial is adjusted to the frequency of rotation at which the testing is
performed.
erformed. Turn on the Strobotac, the frequency controller, and the power supply. Adjust the
frequency control (using the arrow keys) until the white stripe painted on the flywheel seems to
slow and stop. At this point, the frequency of the stroboscope equ
equals
als the frequency of rotation of
the flywheel. The frequency controller displays the frequency of the rotation of the flywheel in
rotations per minute while the shaft is rotating.
4.2 Sample Loading
Loosen the screws on the clamps and remove the tops of the clamps (Figure 1, [4]). Place these
parts on the laboratory table. The sample should be precisely measured and not exceed the length
of one centimeter. Attach the sample to clamps by placing the sample on the clamp bottoms
(Figure 2, [3]) and tightening
ng the screws with supplied the Allen key. Loosen the setscrews
(Figure 2, [1]) and apply tension to the sample by adjusting the outcropping of the load cell.
Tighten the setscrews using the supplied Allen key.
Figure 2: Linear Track - Setscrews [1], Bio
Biological
logical sample [2], Grip bottom [3], Grip top [4]
51
4.3 Power source
The power supply can be used as a current source. The set
set-up
up of this method is described below.
1. Connect the desired circuit to the power supply output terminals.
2. Turn the power supplyy on. The power supply's outputs will be disabled (the OFF annunciator is
on).
3. Enable the outputs by pressing the Output On/OFF key (see Point 8, Figure 3)) The CV and +6V
annunciators will be on to indicate that the power supply operates in the constant voltage mode
and that the +6V display is selected. The display is in the meter mode, i.e. the display shows the
actual output voltage and current. To set up the +25 V power supply, press the +25V key to select
the display and adjust the +25V supply voltage
voltage. Do the same for the -25V
25V supply.
4. Set the display for limit mode by pressing the Display Limit key (see Point 3, Figure 3).
3 You will
notice the LMT annunciator blinking to indicate that the display is in the limit mode. The display
shows the actual voltage
ge and current limit values of the selected supply.
5.
You will notice that the second digit of the voltmeter is blinking. Turn the large knob to set the
desired voltage limit (make sure the LMT annunciator is still blinking).
6. Press the Vol/Cur key (see
ee P
Point 11, Figure 3).
). The second digit of the ammeter will be blinking.
Adjust the desired output current that the current source will supply.
7. To return to the meter mode press the Display Limit (see Point 3, Figure 3)) or let the display
time-out to return automatically to the meter mode. The LMT annunciator will be off3.
Figure 3- Front panel of the E3631A power supply4
3
University of Pennsylvania, Department of Electrical Engineering. ““Basics of Power Supplies -Use
of the HP
E3631A Programmable Power Supply.” Web Mar 27, 2010.
http://www.ese.upenn.edu/rca/instruments/HPpower/PS3631A.
52
4.4 Flywheel Operation
Turn the power to the frequency controller on. Gradually increase the speed of the motor by
pressing the upward arrow key until desired test speed is reached. When the test is complete,
gradually slow the flywheel by pressing the downward arrow key. Turn off the power to the
frequency controller.
5. Maintenance
Preventative maintenance should be performed on the LAHVTR to reduce risk of failure.
Before performing testing on the LAHVTR, inspect the contact pin for chipping or deformation.
Replace the pin if needed.
Dismantle and clean all parts of the LAHVTR exposed to biological tissue. Use warm water and
a disinfecting soap.
4
Agilent Technologies (2000). User’s Guide - Agilent E3631A Triple Output DC Power Supply.
53
Appendix A
54
Appendix F: Engineering Drawings
List of Drawings
(See Attached)
Frame Explosion L-00-00
Frame Explosion L-01-00
1. Bottom Mount L-01-01
2. Horizontal Mount L-01-02
3. Grip Support L-01-03
4. Removable Block L-01-04
5. LVDT Support L-01-05
6. Removable Block [2] L-01-06
7. Vertical Mount L-01-07
8. Bearing Mount L-01-08
9. Bearing Mounts L-01-09
Test Bed Explosion L-02-00
1. Linear Track Block B L-02-01 – Drawing 1
2. Linear Track Block B L-02-01 – Drawing 2
3. Linear Track Block L-02-02
4. Specimen Bath L-02-03 – Drawing 1
5. Specimen Bath L-02-03 – Drawing 2
6. Specimen Grip 2 L-02-04
7. Grip Top L-02-05
8. Tooth Guide L-02-06 – Drawing 1
9. Tooth Guide L-02-06 – Drawing 2
10. Impact Tooth L-02-07
11. Bushing L-02-08
12. Damper Plate L-02-09
13. Damper Rod L-02-10
14. Stain Block L-02-11
15. Guide Shaft L-02-12
16. Housing Stop Plate L-02-13
17. Load Cell L-02-14
18. Tooth Housing L-02-15 – Drawing 1
19. Tooth Housing L-02-15 – Drawing 2
20. Tooth Housing L-02-15 – Drawing 3
Flywheel Explosion L-03-00
1. Flywheel L-03-01 – Drawing 1
2. Flywheel L-03-01 – Drawing 2
3. Flywheel L-03-01 – Drawing 3
4. Engagement Pin L-03-02
5. Main Shaft L-03-03
55